Traditionally, a mobile station's feedback of channel state information (CSI) is designed mainly targeting horizontal CSI. For example, feedback of a precoding matrix indicator and channel quality information for downlink beamforming in long-term evolution (LTE) systems informs a corresponding base station of the horizontal direction along which the mobile station receives the strongest signal and the associated channel strength. However, when active antenna elements are introduced in the vertical domain as well, CSI reference signals have to be designed to include vertical CSI feedback along with horizontal CSI feedback.